1. Field of the Invention
This invention relates to an apparatus and process for producing shaped metal parts from semisolid metal preforms.
2. Description of the Prior Art
Vigorous agitation of metals during solidification is known to eliminate dendritic structure and produce a semisolid "slurry structured" material with thixotropic characteristics. It is also known that the viscosities of such materials may be high enough to be handled as a soft solid. See "Semisolid Metal Casting and Forging", M. P. Kenney et al., Metals Handbook, Vol. 15, 9th Ed., pp. 327-338, Casting, ASM INTERNATIONAL, Metals Park, Ohio, 1988. A pioneer patent broadly describing the concept is U.S. Pat. No. 3,842,895 to Mehrabian et al. which issued on Oct. 22, 1974.
Semisolid metals offer a number of significant benefits. Particularly significant for higher-melting alloys, semisolid metalworking affords lower operating temperatures and reduced metal heat content (reduced enthalpy of fusion). Also, the viscous flow behavior provides for a more laminar cavity fill than can generally be achieved with liquid alloys. This leads to reduced gas entrainment. Furthermore, solidification shrinkage is reduced in direct proportion to the fraction solidified within the semisolid metalworking alloy which, in turn, reduces both shrinkage porosity and the tendency toward hot tearing. Yet an added benefit resulting from the concept is that the viscous nature of semisolid alloys provides a natural environment for the incorporation of third-phase particles in the preparation of particulate-reinforced metal-matrix composites. In this instance, the enhanced viscosity of semisolid metalworking alloys serves to entrap the reinforcement material physically, allowing time to develop good bonding between the reinforcement and the matrix alloy.
However, processes for producing shaped parts from such slurry structured materials, particularly on a continuous basis, present a number of problems. Such processes require a first step of reheating a slurry structured preform charge to the appropriate fraction solid and then forming it while in a semisolid condition. At an earlier time, a crucible had been considered essential as a means of containing the material and handling it from its heating through its forming cycle. However, the use of such crucibles was recognized as costly and cumbersome and furthermore created process disadvantages such as material loss due to crucible adhesion, contamination from crucible degradation and untoward chilling from random contact with crucible side walls. Other problems are similarly involved in the heating, transport and delivery of preforms which are in a semisolid condition. Accordingly, a process was sought which would provide considerable manufacturing economy, particularly a process which would not require crucibles or other containing means and which is capable of operation on a continuous basis.
Such a process is disclosed in U.S. Pat. No. 4,569,218 which issued to Baker et al on Feb. 11, 1986, the entire disclosure of which is incorporated herein by references. As explained in that patent, it was found possible to produce on a continuous basis shaped metal articles from slurry structured freestanding metal preforms by sequentially raising the heat content of the preforms as they are passed through a plurality of induction heating zones. The heating sequence was such that it avoided melting and resulting flow and permitted thermal equilibration during transfers from one zone to the next as the preforms were raised to a semisolid temperature. That invention provided preforms which were substantially uniformly semisolid throughout. The freestanding semisolid preforms were then transferred to a press or other shaping station by means of mechanical transferring means which gripped the preforms with a very low force. This construction served both to prevent substantial physical deformation of the semisolid preform and reduced heat loss. The transferring means were also heated if desired to even further minimize heat loss of the preforms during transfer.
Notwithstanding the substantial advances which were presented in the patent to Baker et al., some problems have persisted with the process as it was then known. For example, with placement of the metal preform in the press with the ram positioned contiguous to the preform in preparation for the forming operation, the ram undesirably served as a heat sink substantially lowering the temperature of the preform. Such temperature reduction of the preform altered the characteristics of the resulting article and thereby impaired the benefits sought to be obtained by use of a semisolid preform. In order to avoid this condition, the natural inclination was to withdraw the ram a substantial distance from the preform immediately prior to the forming operation. In this case the end of the ram was preset to a relatively generous distance from the article before the ram was activated for the forming step. This was done manually by a set up man who had to physically place himself in and around the press to set limit switches in combination with the position of the ram. This set up procedure is dangerous, very time consuming and cannot be carried out very precisely. In addition, this manual approach to correcting for the earlier described situation has other serious drawbacks. Specifically, air in front of the ram is captured by the ram and forced into the interstices of the metal preform as it enters the cavity of the tooling used to form the final article. This can have a deleterious effect of a resulting article having air field voids in its structure. Such articles would, as a result, be weaker than solid articles having the same configuration.
Another undesirable occurrence is possible when the ram has been withdrawn a substantial distance from the preform immediately preceding the inception of the preform operation. In this instance, as the ram advanced rapidly into engagement with the preform, there was often sufficient deformation of the preform to enable some of its substantial liquid content to splatter outside of the cavity of the tooling, thereby reducing the volume of the formed article. The volume of the preform is precisely determined in order to arrive at a formed article which is solid and complete in all respects. Hence, if material present in the preform does not find its way into the cavity of the tooling, the finished article will either have voids in it or it will be incomplete. In either event, the resulting article will not be satisfactory for its intended purpose.